1. Field of the Invention
The present invention generally relates to automobile final drive assemblies for transferring engine power to the drive wheels of an automobile. More specifically, this invention relates to a rear axle differential drive assembly which utilizes a worm gear system consisting of an input worm and an output worm gear, with a differentiating clutch mounted adjacent of the output worm gear to provide differential operation of the rear wheels of the automobile.
2. Description of the Prior Art
Automobiles conventionally are equipped with a differential final drive to permit the drive wheels to rotate at different speeds. This capability is necessary when the vehicle is cornering, such that the outer drive wheel must cover a greater distance than the inner drive wheel. Generally, a conventional differential final drive includes a set of gears with three rotating members (one input and two outputs), each having a speed and torque relationship to the other. In a rear wheel drive, the first rotating member is an input gear driven by the drive shaft of the engine. The input gear drives a bevel ring gear having an axis of rotation generally perpendicular to that of the input gear. The bevel ring gear is formed on the perimeter of a differential case, which typically carries four pinions mounted to an X-shaped cross-shaft pinion pin mounted transverse to the axis of rotation of the bevel ring gear. The four pinions simultaneously mesh with a pair of bevel gears secured to two drive shafts, both of which are coaxial with the bevel ring gear's axis of rotation. One of the drive shafts extends through the bevel ring gear to the first drive wheel, while the second extends in an opposite direction to the second drive wheel.
On a straight course, the entire differential case rotates in unison, such that both drive wheels rotate at the same speed so as to maintain a 1:1 torque relationship between them. The pinion gears do not rotate on the cross-shaft pinion pin, but merely provide direct engagement between the bevel ring gear and the bevel gears on the drive shafts. However, during cornering, the pinions will rotate as necessary so as to permit the drive shaft coupled to the outer wheel to rotate faster than the drive shaft coupled to the inner wheel. In effect, the pinions serve to establish an equilibrium of torques and forces between the drive wheels.
While the above description covers the very basic operation of most differentials currently in production, other differential gear systems have been proposed. One such system is a worm and worm gear combination. Worm gears are a desirable alternative to the more conventional bevel gear/pinion arrangement in that they provide for a quieter and smoother gear reduction, with a large contact area between the worm thread and the worm gear teeth that promotes a high load capacity. Worm gear systems are also desirable in that they are capable of very large reduction ratios for a given center distance between shafts. Accordingly, worm gear systems can provide an extremely compact gear reduction capable of high speeds and torques. Compactness is a highly desirable feature in automotive applications where size, weight and ground clearance are important considerations.
Examples of worm gear differentials known in the prior art include U.S. Pat. No. 1,322,392 to Atwood, U.S. Pat. No. 1,536,112 to Lindgren, U.S. Pat. No. 1,704,861 to Lewis, U.S. Pat. No. 2,043,006 to Morgan and U.S. Pat. No. 4,128,021 to Knowles. Each of these prior art worm gear differentials generally teach the use of a worm which is driven by the vehicle's drive shaft. The worm meshes with a worm gear formed on a differential case. At least one, and more often both, of the wheel drive shafts are driven by the worm gear through a suitable mechanism that provides differential operation between the drive wheels. For example, the differential mechanism taught by Morgan is a more or less conventional differential bevel gear/pinion arrangement. However, as a result, the differential taught by Morgan is rather large and complicated in its construction due to the number of components required.
In contrast, Atwood, Lindgren, Lewis and Knowles teach the use of a slip clutch member mounted on at least one of a pair of drive axles for providing the desired differential operation of the drive wheels. Lindgren, Lewis and Knowles rely on engagement features formed on the clutch members which engage specially adapted surfaces of the worm gear, necessitating a worm gear which is structurally complicated in its design and construction. In contrast to the engagement features taught by Lindgren, Lewis and Knowles, Atwood teaches the use of a single frictional slip member which engages an annular recessed surface formed in the worm gear, which again results in a somewhat complicated worm gear form. The final rear drives taught by the above prior art are further complicated because the drive axles and the worm gears are maintained concentric with each other by independent bearing supports. Accordingly, the prior art differential mechanisms increase the overall size of the entire final drive assembly. As a result, the desirable compactness of a worm gear system is lost to some degree with worm gear differentials known in the prior art.
Furthermore, independent support of the worm gear and the drive axles results in a less rigid final drive assembly, in that the worm gear and the drive axles are supported by different sections of the differential housing and offer little, if any, structural support to each other. Rigidity and stiffness in a worm gear system is necessary to inhibit deflection and torsional displacement of the worm gear relative to the worm, particularly in view of the significant forces generated between the worm and worm gear which act to separate them.
Accordingly, what is needed is a compact automotive final drive assembly which incorporates a worm gear system for purposes of minimizing the size of the final drive assembly, so as to minimize weight and provide for maximum ground clearance, wherein the worm gear system utilizes an uncomplicated worm gear configuration which, in cooperation with the drive axles, enhances the structural rigidity of the final drive assembly.